Couple of questions

1. Don't Instants go to your graveyard RIGHT after you use it??
2. You can't use Instants on other people's turn, right?
3. What does Rampage do? I have a card, a summon legend, and in the text it says Rampage: 1, then the rest of the text after that. What does it do?

1) First instants are cast and have a chance to be inturrupted by another spell. Then the spell resolves and all the abilities of it go through. Then it goes to your gravyard.

2)Yes, that is the whole point of instants.

3)Rampage means it get's +X/+X for each creature blocking it beyond the first(where X is the number after Rampage.) So if your Rampage creature was blocked by 2 creatures it would get +1/+1 untill end of tunr.

You cast spell X, Spell X goes on the stack.
Your oppenent can respond, he casts Spell Y, Spell Y enters the stack.
You get to respond, You don't.
[At this point, your opponent can also respond, assume he does not for the purposes of this example. But _both_ players must pass an empty stack before anything starts resolving. --Zadok001]
The stack starts to resolve. Spell Y happens. Spell Y hits the graveyard.
Your spell now resolves (unless y does something), Spell X happens. Spell X hits the graveyard.

Now in the case of buyback, AFTER the spell resolves and would go to the graveyard, it instead goes back to your hand. This is important cause if you cast forbid and then forbid if countered you can't cast forbid again cause it hasn't resolved.